Polyester resin systems are useful in a multitude of molding and coating applications. Of particular interest herein are polyester resin systems having components and additives which allow curing at low energy levels, provide improved shelf life and reduce gel-time drift.
Important parameters that desirably should remain fairly constant for the useful life of a resin system during a storage period, or its "shelf life," are the gel and curing times of the resin system. Variations in the reaction time for gelling, and subsequent curing, of a polyester resin system during storage may be characterized as "gel-time drift." These variations are typically measured as the difference between a gel time after a period of storage and a gel time just after formulation of the resin system. Usually, polyester systems in storage for long periods acquire longer gel or curing times. Some polyester systems, however, may, after long storage periods, exhibit curing times which are shorter than the initial curing time of a freshly manufactured batch. Typical commercially-available polyester systems often have gel-time drifts in the range of minus 50 percent to plus 200 to 300 percent. Such variations frequently cause intolerable problems during molding or coating application processes where predictability of gel time is a necessity.
Gel-time drift of a polyester resin system presents a complex problem inasmuch as there may be several interrelated factors responsible, namely, physical parameters of resin formulation, chemical composition of the resin system, the presence of contaminants, and shipping, handling and storage conditions. The problem of gel-time drift is particularly acute for the more highly reactive resin systems which may contain chemical promoters for accelerating the rate of gelling or for lowering the temperature of reaction as may be required in a casting operation. It has been found that a promoted polyester system may not only exhibit large variations in gel-time drift, but gel-time drift may vary widely between samples within a single batch of a polyester formulation due to different storage and handling conditions.
There have been several additives found for controlling or suppressing gel-time drift in polymerizable molding and coating compositions, most of the compositions being of the polyester type. Some of these additives are metallic salts such as the alkaline-earth metal salts disclosed for use with polyester resins in U.S. Pat. No. 3,367,905; or the additives comprise organo-metallic salts such as phthalimide-potassium salts for use as stabilizers disclosed for use with phthalate-polyol transesterification products in German Offenlegenschrift No. 2,556,317. Other compounds have been used to compensate for the presence of special components. For example, U.S. Pat. No. 4,088,710 discloses that gel time may be reduced for an unsaturated polyester thermosetting resin, which contains a gel-time lengthening vinylphosphonate flame retardant, by adding an effective amount of maleic anhydride to the polyester. In British Pat. No. 1,281,891, unsaturated polyester resin containing a cobalt salt promoter is disclosed as stabilized against gel-time drift by the addition of bis(2-hydroxyethyl)sulfide. Certain amine-containing compounds have been used as gel-time drift stabilizers. For example, improvements in stability and gel-time drift have been found for polyester resin incorporating a combination of two or more quinone-type inhibitors with an aryl ethanolamine, as disclosed in Werts, A. P., Annual Conf. of Soc. Plast. Ind., Reinf. Plast./Compos. Div., Proc., 26th, 1971, 4-C, 1-10. A decrease in gel-time drift has been reported for a polyester resin system containing a cobalt-dipyridyl promoter combination [Goodman et al., J. Paint Tech., 43, 59 (1971)]. It was noted in Kozyreva, H. G. et al., Zh. Prikl. Khim. (Leningrad), 1976, 49(4), 920-1, that the degree of cross-linking in unsaturated polyesters may be affected by the presence of an intiator system containing aminoesters of methacrylic acid. In U.S.S.R. Pat. No. 535,317 to Saifullin et al., there is disclosed a polyurethane prepolymer composition having improved storage stability, which composition comprises free NCO groups and a catalyst provided by a solid complex salt of a tertiary amine and a carboxy-containing compound selected from polyester acids and acids having dissociation constants in a range of 1.times.10.sup.-3 to 6.times.10.sup.-6.
The problem of a gel-time drift is particularly acute for polyester resin systems containing a number of chemical-or-physical property-altering additives in combination, such as initiators for initiating curing, reaction-rate promoters for accelerating curing and inhibitors for preventing premature curing of a polyester resin during storage. Also, the presence of fillers, pigments, dyes, thixotropic additives, reinforcement materials, flame retardants, waxes, or lubricants may further contribute to gel-time drift in an unpredictable or uncontrollable manner. There remains, therefore, need for methods of treatment which will reduce gel-time drift and stabilize curing behavior in polyester resin systems, especially in those polyester resin systems containing several property-altering additives.